Simulating Lake and Wetland Areal Coverage and Numbers under Scenarios of Future Groundwater Recharge: Lake Mega-system of the Nebraska Sand Hills

Abstract:

Integrated groundwater (GW) and surface water (SW) systems in arid parts of the world respond to and recover slowly from climate changes. This creates multi-generational issues with dramatic economic, social, and ecological consequences, and could likely be the case in the future for the High Plains aquifer. Despite the semi-arid climate of the Nebraska Sand Hills (NSH), groundwater recharge (GR) rates are the highest in the entire High Plains aquifer region, mostly because of thick, highly permeable sand dunes. Along with the large capacity aquifer, the GR rates in the NSH contribute to relatively steady stream baseflows and a shallow water table, creating a very complex system of several thousands of shallow (about 0.8 m on average) closed-basin lakes, and wetlands, integrated with GW.

To explore the GW dynamics and address the possibility, and uncertainty range of impacts caused by future climate change on surface water, we quantified potential changes to GR rates resulting from three thoughtfully-selected Global Circulation Model (GCM) projections from the WCRP CMIP3 archive. Cumulative future period GR (as the difference between precipitation and evapotranspiration) was used as the criterion to select GCM runs, including total cumulative GR (from 2010 to 2099) nearest to the median (±1 standard deviation). This allows for: determining the most likely GR estimates, characterization of uncertainty, retaining the temporal variability of individual GCMs, while reducing time needed to perform hydrological simulations. Future GR changes from the three selected GCM runs were averaged by decade and used to force transient groundwater model runs within a 40,000 km² part of the NSH. Dynamics of lake and wetland spatial distribution and total areal coverage and numbers, were simulated with the GW model from 2010 to 2099. Addition model runs, with fixed end-of-century GR rates, elucidate response time required for the GW system to reach equilibrium under the changed climate regime.